In a piston for a positive-displacement, reciprocating piston-in-cylinder device, such as an internal combustion engine prime mover or a pump, the (upper) part of the piston nearest the working fluid commonly incorporates a coolant gallery, for a coolant, or more specifically (fluid) heat transfer medium, typically a liquid, such as a lubricating oil. For a cast piston, such a coolant gallery can be integrally cast within it. This is typical of current aluminum alloy pistons for medium-duty diesel engines.
In striving for (energy conversion and thermodynamic) efficiency, reduced emissions and enhanced “user satisfaction”, internal combustion engine design must balance conflicting requirements. The materials used in the construction of such engines are under severe stress and there is little margin between a robust, cost-effective design and one that will have insufficient durability. Reduced size and weight is a key benefit for customers, yet increased power is also often required.
A fundamental limit upon the compression ratio of a spark-ignition, gasoline engine, and hence its thermodynamic and fuel combustion efficiency, is the phenomenon of pre-ignition, or “knock”, that is, uncontrolled explosion, rather than progressive timed combustion. The destructive effect of knock is well-known, and much effort has been expended in its resolution. In gasoline engines, the influence of piston temperature upon pre-ignition and knock is relatively minor, but well-known.
Generally, any reduction in combustion chamber temperatures will directly influence fuel combustion efficiency. Compression-ignition (diesel) engines do not suffer the severe problems of preignition or knock attendant spark-ignition, gasoline engines and so they can be made in much greater sizes and run at much higher levels of super-charge. However, the high compression ratios employed by diesel engines for higher thermodynamic and fuel combustion efficiency have led to diesel engine pistons needing sophisticated piston cooling systems. This has long been recognized and prompted a plethora of designs.
Until the advent of finite element stress analysis, the extremely complicated thermal and mechanical stresses in pistons could not be effectively calculated and so piston designers had limited formal (quantifiable) guidance. Many complex and imaginative solutions were tried, but few were successful. Also, the cast aluminum alloy piston continued to be superior and less expensive in smaller engines. However, the problem of piston temperature remained.
Component cooling around the working fluid is a trenchant problem. Component temperatures need to be kept low because most materials suffer a reduction in strength at elevated temperature. The coolant also degrades if the wetted surfaces become too hot. High thermal gradients in components, arising from intensive heating and cooling, also produce high thermal stresses. Increased engine rating exacerbates this problem considerably and much attention has been devoted to improving component cooling.
A piston is closest to the working fluid and the intense heat of combustion and is thus the component most vulnerable to thermal and mechanical stresses and shock. Piston structures suffer localized extreme temperature gradients and working pressures. The risk of material failure due to overheating can be eased by the provision of effective internal piston cooling. In that regard, a piston represents a key engine component and as such is a major contributor to performance and reliability. Consequently, in piston engine development, piston temperature and hence piston cooling has long been an important issue.
Designers of larger engines, where component cost is less of an issue and the greater size allows more design freedom, frequently multi-piece pistons are utilized, often with steel crowns. These crowns often have complex geometries to provide cooling where it is most needed and a temperature profile that is carefully calculated to give the longest life and optimum engine performance. Some (e.g. as described in 1981 CIMAC paper 0109) have used a ring gallery (created by the space between crown and body), together with a series of drilled blind or closed-ended holes. The ring gallery disposition allows coolant fluid (such as lubrication oil) to come close to sensitive or vulnerable areas of the piston, i.e. where adverse temperatures and thermal stresses are most acute or less readily accommodated.
Blind holes do not allow fluid to flow in the normal (e.g. coherent uni-directional, continuous, closed-loop, re-circulatory) sense. However, because of the severe accelerations experienced by the piston in its reciprocating motion, coolant fluid is thrown into and out of the holes upon each piston reversal and hence has high, albeit intermittent, flow velocities, in relation to the sides of these blind holes, thus promoting heat transfer.
For smaller engines where initial cost (i.e. original manufacturing, as opposed to service-life) is more important and space is limited, hitherto known blind-hole coolant gallery configurations have proved impractical for the majority of applications.
Many minor modifications to galleries have been proposed hitherto, with specially shaped entrances and exits, tilted axes, convergent or divergent walls, etc. but none of these have achieved a significant increase in overall surface area for heat transfer through a coolant medium.
In one approach an oil jet projecting oil at the underside of the cast aluminum piston was the easiest and lease expensive solution, but one which only increased the allowable rating by some 25-30%. Multi-piece piston of relatively simple architecture were devised with one or two substantially circular cavities, through which oil could be passed. These pistons succeeded where the more complicated versions had failed. This was largely due to a simple architecture and generous profile transitions or end radii which inhibited initiation of thermal cracking. These pistons had less effective cooling than many more complex designs, and so operated at higher temperatures, but their simplicity of construction entailed lower stress levels.
Latterly, with the advent of finite element (FE) stress analysis techniques, some more complex features were reintroduced, but with the benefit of a computational tool allowing modeling and evaluation of the implications of design proposals before manufacture. Single-piece, cast pistons were also developed, incorporating more complex cooling features than merely an under-crown oil jet.
Simple “open gallery” designs 50 such as depicted in FIGS. 4A through 4C where cavities were cast in above the piston pin bosses gave a modest, but still useful, increase in rating capability (circa 15%) because the oil had a greater wetted contact surface area over which to extract heat. The oil supply was again by standing jet, and the galleries were virtually emptied at every bottom-dead-center (BDC), by high piston acceleration.
Another approach was a “cooling coil” design 55 such as depicted in FIGS. 5A through 5C, in which a copper or steel tube 56 was coiled into a spiral and cast into the piston body. Holes for oil feed and drain were provided, and coolant (typically oil) was fed up a passage or oil-way (drilled) in the connecting rod and, either by a slipper arrangement up a hole at the center of the under-crown (as shown in FIGS. 5A and 5C), or by a fairly tortuous route, via the piston pin and (cast and/or drilled) passages, through the pin boss.
Experimentation showed that the heat transfer coefficient of the piston/oil interface was at its greatest when the oil only partially filled the cavity in the piston, and was thrown violently against the walls of the gallery by piston acceleration. Such a “cocktail shaker” approach became a standard technique for oil cooling and coolant channels filled with oil gradually died out.
The narrow channels of a cooling coil could not be run only partially-filled, because the oil flow-rate required to carry away the heat flow could only be sustained in such narrow passages by filling them with oil. Thus, although they could be produced with somewhat increased surface area, as compared with, say, a single toroidal gallery, cooling-coil pistons were not pursued.
Instead, for highly rated engines with aluminum pistons, a generally toroidal gallery with jet feed into a drilled inlet were utilized. This is depicted as a “full gallery” piston 60 in FIGS. 6A through 6C.
A variant is a “horseshoe gallery” piston design 65, such as depicted in FIGS. 7A through 7C, where oil flows only one way around the piston, from inlet to drain, rather than splitting and travelling in both directions.
Many, many different features have been tried on galleries to increase their efficiency, but without an analytical tool capable of predicting the flows at a detail level, there was little prospect of progress, except by accident. Nevertheless, certain successful features addressed critical factors such as the temperature of the top ring groove 185 in FIG. 8B, (because of oil carbonisation); the combined thermal and mechanical stress at the edge of the combustion bowl 189 in FIG. 8B, and the combined stresses around the gallery (principal compressive stress) shown as 188 in FIG. 8B. Also, the dimensions 181, 182, 183 and 184 around the gallery(s) require careful selection and control for a robust design.
FIG. 8A shows a known coolant gallery configuration developed by Associated Engineering and adopted in Japan. Although the gallery 82 is not large, by making it from a fabrication attached to the back of the top ring insert 81, the temperature at the top ring groove 86 is reduced. The close proximity to the sensitive area of the combustion bowl edge 89 also enables this gallery to reduce the temperature significantly at this point. Feed and drain holes 83 usually have to be drilled at an angle, because of the limited space available. The limited surface area available for heat transfer means that the bulk piston temperature is not reduced as much as is possible.
FIG. 9A shows a localized (entrapment or capture barrier) “weir” 93 used around the junction of a gallery 91 and a drain passage 92 to prevent the gallery 91 emptying of oil at every bottom dead center and also when the engine is stopped. This feature was commonly adopted, but careful sizing of inlet and drain holes, to match them to the gallery size and the oil flow rate, has made this feature redundant.
FIG. 9B shows a “swept bend” inlet hole 102, together with a diffuser 103, before the oil enters the main gallery at diameter 101. The effectiveness of this proposal is unknown, but it could be useful to harness the high velocity of the jet (typically around 20 m/s) in order to enhance the oil velocity along the walls of the gallery.
FIG. 9C shows a typical inlet, with conical section 113 at the entrance of a feed passage 112 to a gallery 111. This is an attempt to capture an (oil) jet, even if it is somewhat divergent, or cannot be aimed straight at the entrance at all piston positions, as the piston travels up and down the cylinder. It is commonly used on many of the jet-fed galleries.
Many of the features described can be used together, and there are many more that can be included. Also, current developments of computational fluid dynamics are becoming capable of calculating the flows of oil and heat in a piston coolant gallery and thus can analyze the effect of geometric variations.
In general, the important factors that influence coolant gallery effectiveness are the mean oil velocity at the surface; the gallery wetted area; the gallery position (mean heat path from source to oil); the gallery surface condition; and the coolant (oil) properties. Other major factors influencing piston temperature include the mean in-cylinder gas temperature; the piston crown area; the piston crown surface heat transfer coefficients (dominated by gas velocities and mean cylinder pressure); and the heat transfer coefficients to cylinder walls.
Although many complex shapes have been proposed for machined coolant galleries, in multi-piece pistons, these have all had to be readily reproducible by (selective material removal) tooling, whether cutter, spark-erosion or chemical milling.
Pistons of aluminum alloy, with cast in coolant galleries, are well established. Indeed, the majority of pistons are made of aluminum alloy, because of its all-round cost-effectiveness.
Cast galleries have tended to be very simple, partly because of the limitations of the foundry processes, and also because of the dangers of introducing stress raisers. Any deviation from a simple form will raise stresses; those deviations lying substantially perpendicularly to the principal stresses having the greatest effect. Foundry processes are also such that changes in section are always accompanied by the danger of porosity, “cold-shuts”, and other similar defects that effect the integrity and strength of the metal locally. Hitherto, particularly in cast pistons, the coolant gallery has remained configured as generally a relatively crude heat-transfer system.
The usual method of manufacture is to use a water-soluble core of salt, which is placed in a die, prior to pouring molten aluminum alloy. Early processes used a mixture of salt and foundry resin (such as is commonly used with foundry sands); the resin being thought necessary to bind the grains of salt together. Foundry process development recognized that the salt grains would bind together successfully, if pressed together at moderate pressures, and also gain some more strength, if the cores were sintered at elevated temperature. Thus the salt cores could be made more accurately, with less so-called “out-gassing” arising, since foundry resins produce gas, when exposed to the molten metal. This allowed successful casting of finer and more intricate detail in piston features.
In a foundry casting process, after the piston has cooled, the core is washed away with a high pressure jet of water which rapidly dissolves the salt. This leaves a (through) hole or pocket (to form an intended coolant gallery or passage), within and/or through which a suitable coolant fluid, such as lubrication oil, can be passed, when operating an engine in which the piston is installed.
Incorporation of a coolant gallery into the piston entails some additional cost, but its overall cost-effectiveness is witnessed by its widespread adoption in highly-rated diesel engines, where piston temperatures would otherwise pose a problem.